The use of x-ray diffraction techniques for measuring residual stresses in crystalline substances such as metal or ceramic materials is well-known. The general idea with the use of x-ray diffraction is to subject the material to the radiation of x-rays with the resulting sensed x-ray diffraction peak interpreted to arrive at a measurement of a strength related characteristic, i.e. stress, retained austenite, hardness of the part material, to show, for instance, the level fo fatigue in the material.
More particularly, the present invention relates to open beam type x-ray diffraction equipment that utilizes a cantilevered x-ray goniometer head having fiberoptic detectors carried toward the forward end of the head. In contrast, there are x-ray diffraction systems that are of a closed loop variety in the sense that the x-ray head is positioned at one location along a circular mount with the detectors spaced generally across from or diametrically opposite to the x-ray head along the circle mount with the part inserted in the space therebetween. In these systems, part size is limited due to this orientation of the x-ray head and detectors, and generally, coupons have to be taken from the part that is desired to be measured. With the open beam approach, coupons do not have to be cut out from parts since the x-ray head and detectors are integrated with each other. However, current open beam x-ray equipment still suffers from shortcoming as described below.
One such problem is that there is no open beam type x-ray apparatus that can perform these types of measurements on a wide variety of different parts and/or different materials or materials with different characteristics such as with respect to crystalline structure. Generally, the size of the goniometer or x-ray tube head relates to the power required for its operation. With greater power levels, the diameter of the x-ray tube is larger for heat dissipation purposes. The power for a goniometer head is selected to generate sufficient x-ray flux for the x-ray diffraction process to take place with particular materials or material characteristic.
The problem with the use of larger diameter x-ray heads for taking measurement is that with certain parts such as pipes and the like, it would be desirable for measurements to be taken of the material in the interior of the part. Depending upon the relative size of the inner diameter of the pipe and of the head on the x-ray diffraction apparatus, it may be physically impossible for the x-ray head to fit inside the pipe and take a suitable measurement. Also, where part surfaces are in confined areas such as in close confronting relation to each other as can be found on fillets of aircraft rotor disks at the roots of the rotor blades, set-up of the x-ray diffraction equipment to precisely direct the x-rays at the surface location from which a measurement is desired can be difficult, and is usually unwieldy where the large x-ray head itself has to be manipulated. Since current open-beam x-ray diffraction units have x-ray heads that are specifically tailored to a material or materials from which measurements are to be taken, many different sizes and types of x-ray diffraction units generally are necessary to take measurements on a wide range of different parts that are of different or materials or material characteristics, and/or having different configurations raising equipment costs accordingly. Thus, there is a need for an x-ray diffraction system and method that allow for greater flexibility in terms of the different types of parts and part geometries from which accurate x-ray diffraction measurements can be taken.
Another problem in using this equipment is the measurement precision that is desirable, and the issues this creates with the system's drive mechanism for pivoting or rotating the tube during a measurement operation. During x-ray measurement operations, the tube is typically pivoted to vary the position of the x-ray emitter or collimator from which x-rays are emitted toward the part to obtain more precise measurements by way of sampling techniques as opposed to keeping the tube and its collimator fixed relative to the part. As mentioned, the tube is generally cantilevered and is pivoted back and forth along a fixed arcuate rack by a motor drive including a pinion gear which pivots with the tube. In another configuration, the motor pivots the rack which is fixed to the tube. In both instances, the motor is also part of the cantilevered structure of the current x-ray diffraction units. Thus, current x-ray diffraction units have heavy cantilevered weights, particularly those having larger x-ray tubes. Since the x-ray diffraction techniques employed rely on distinguishing minute differences in the diffraction peaks and patterns of the detected x-rays, precision is required for pivoting the x-ray head. Inaccuracies can be created in present drive mechanisms with transmission belts that stretch and/or with backlash problems that occur between meshed gears due to play therebetween such as with the above-described rack and pinion arrangement. Therefore, there exists a need for a drive mechanism that provides for precision movements of the x-ray head for taking efficient and accurate measurements therewith.
Various part sizes and configurations pose yet another problem for standard x-ray diffraction measurement techniques in that the preferred measurement technique, d v. Sine2 ψ, cannot be used to measure all part configurations. When using this technique, the sensors are positioned such that they remain in a plane that is parallel to the plane of angular rotation of the head itself. This technique is the most accurate way to measure strength related characteristics of parts because of the geometrical relationship between the x-ray emitter, part, and sensors. However, this technique requires enough room to allow the head to oscillate back and forth without having the sensors hit the part itself. Therefore, there are situations where a different method of measuring, called d v. Sine2 χ, must be used. When using this technique, the sensors are in a position that is shifted by ninety degrees about the longitudinal axis of the emitter from the d v. Sine2 ψ configuration so that the sensors are generally aligned or parallel to the longitudinal axis of the x-ray tube. Then the head rotates as it normally does during x-ray diffraction measurements. This sensor configuration allows the user to take measurements in narrow places such as between the roots of blades. However, utilizing the d v. Sine2 χ technique requires a sacrifice in measurement accuracy. Currently, one has to switch x-ray diffraction apparati in order to change from one technique to another. Accordingly, an x-ray diffraction apparatus that has flexibility in terms of the measurement techniques it employs would be desirable.